Stable myocardial hypertrophy can be produced in cats by 4.0 mm I.D. bands around the pulmonary artery. Employing this model the present study will determine 1) effect of exercise on the myocardial response to increased afterload, 2) whether differences in function exist between myocardium hypertrophied by exercise and that hypertrophied by pressure loading. Myocardial hypertrophy will be produced in cats by treadmill exercise. After hypertrophy develops myocardial function will be assessed in one group by determining length-tension characteristics, force-velocity relations and isometric force development in the right ventricular papillary muscle preparation. In the other group myocardial function will be determined after 6 and 24 weeks of banding. An additional group will undergo repeated periods of exercise beginning 24 weeks after banding and myocardial function will be determined at intervals thereafter. Data in the exercised banded cats will be compared to that in non-exercised cats banded for comparable periods. A new model will be developed in which the stimulus for hypertrophy is gradually applied resulting in chronic congestive heart failure after a period of stable hypertrophy. Pulmonary artery banding in kittens with clips which produce initially only mild constriction will be employed. Removal of the band after development of heart failure and measurement of myocardial function at intervals thereafter will provide information as to whether myocardial function can return to normal once failure has occurred. In the same model digitoxin will be administered during the stage of stable hypertrophy to determine whether "prophylactic" digitoxin affects the rate of development of heart failure.